You are correct, Verizon can do this leaglly (unless the FCC ever gets their act together), but not for the reason you mention. There is still a contract, with agreements about what services will be provided, and how much those services will cost. Unlimited data is one of those services. The "contract period" is simply the minimum length of time the contract will be in effect without the customer having to pay an early termination fee. If Verizon wants to change any terms of the service (throttling, no unlimited, etc) they can do so with 1 months notice to the customer (which they are doing, with about two months to go), regardless of the "contract period". If an ETF were applicable (which none are, since all unlimited plans are well over two years old at this point), then the customer could ditch the contract (and thus cell service) without paying the ETF.
Now, just because they can do it doesn't mean that they should (customer satisfaction). But I'm sure they've done market research to suggest that they will get far more "shared data" plan conversions than they'll lose from upset unlimitted customers.